[Bug c++/123613] [reflection] internal compiler error: tree check: expected record_type or union_type or qual_union_type, have template_type_parm in lookup_base, at cp/search.cc:265

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123613

--- Comment #3 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:007969e7765343619925f726ec1f340895c192b3

commit r16-8292-g007969e7765343619925f726ec1f340895c192b3
Author: Marek Polacek <polacek@redhat.com>
Date:   Thu Mar 19 13:10:38 2026 -0400

    c++/reflection: ICE with substitute and undeduced auto [PR123613]

    In substitute7.C we have

      template <typename T, auto ... Vs>
      constexpr auto construct_from = T{Vs...}; // #1
      struct Inner {};
      struct Outer { Inner m; };
      constexpr auto r = substitute(^^construct_from,
                                   { ^^Outer, ^^construct_from<Inner> });

    which crashes because the auto in #1 hadn't been deduced when we
    called eval_can_substitute -> lookup_template_variable when processing
    the substitute call.  We can call mark_used to resolve this, because
    it has:

      if (undeduced_auto_decl (decl))
        maybe_instantiate_decl (decl);

    and to make the json-parser.C test work, we have to do something
    similar in eval_substitute, otherwise we crash due to another
    undeduced auto.

            PR c++/123613

    gcc/cp/ChangeLog:

            * reflect.cc (get_reflection): Call mark_used.
            (eval_can_substitute): Don't resolve_nondeduced_context here.
            (eval_substitute): Call lookup_and_finish_template_variable instead
            of lookup_template_variable and finish_template_variable.

    gcc/testsuite/ChangeLog:

            * g++.dg/reflect/json-parser.C: New test.
            * g++.dg/reflect/substitute7.C: New test.
            * g++.dg/reflect/test.json: New test.

    Reviewed-by: Jason Merrill <jason@redhat.com>